1. Technical Field
The present invention relates to the field of polyamide resin technology, more specifically, to a method for preparing semi-aromatic polyamide with low wastewater discharge.
2. Description of Related Art
Aliphatic polyamide, such as PA6 and PA66, boasts excellent mechanical strength, thermal resistance, chemical resistance, abrasion resistance, self-lubrication and low friction coefficient. It has been widely applied in electronic appliances, automobile components, furniture, building materials and fibers, and is one of the most important engineering plastics.
Semi-aromatic polyamide, as one of aromatic polyamides, is a polyamide resin prepared by means of polycondensation of aromatic diamine or dicarboxylic acid with aliphatic diamine or dicarboxylic acid. Since an aromatic ring is introduced into the main chain of polyamide molecules, the thermal resistance and mechanical property are improved, water absorption is reduced and a proper performance price ratio is achieved. It is a kind of resin with high thermal resistance between general engineering plastic nylon and high-heat resistant engineering plastic PEEK, mainly used in automobile and electric & electronic industries. With the rapid development of high technology, its applications achieve new breakthrough and progress and the market demand shows the trend of ascending. Semi-aromatic polyamide products mainly include polyamide MXD6, 6T/66, 6T/61, 6T/6I/66, 6T/M-5T and 9T.
Patent JP57200420, JP58111829, EP1074585A1 and CN1624021A disclose the methods for preparing polyamide MXD6. According to these methods, add aromatic diamine into fused aliphatic dicarboxylic acid by drops, raise the system temperature to remove the water generated due to condensation for polymerization. CN1451677A discloses a method for solid state polymerization of polyamide MXD6, wherein the polyamide is stored in specific conditions, and the yellowness of polyamide MXD6 prepared is very low even it takes 20 days or longer from the completion of preparation of the primary polyamide to solid state polymerization.
Since the melting point of polyamide 6T exceeds the decomposition temperature, a third monomer must be introduced to decrease the melting point. Polyamide 6T copolymer is obtained by means of polycondensation of dicarboxylic acid constituent composed of terephthalic acid and isophthalic acid or adipic acid and diamine constituent mainly composed of 1,6-hexamethylene diamine. The relatively high concentration of amide group in polyamide 6T copolymer may lead to poor chemical resistance, water absorption resistance and stability of melt-processing of polymer. The introduction of a large amount of the third monomer reduces the crystallinity of polymer, and may also lead to the decrease of thermal resistance, chemical resistance, water absorption resistance and dimensional stability.
U.S. Pat. No. 5,516,882, U.S. Pat. No. 5,981,692 and U.S. Pat. No. 962,628 disclose the methods for synthesizing polyamide 6T copolymer through melt-polymerization at the high temperature over 300° C. by using terephthalic acid, isophthalic acid, 1,6-adipic acid, 1,6-hexamethylene diamine and 2-methyl-1,5-pentamethylene diamine as main materials. U.S. Pat. No. 6,140,459 discloses the synthesis of polyamide 6T copolymer through melt-polymerization by using terephthalic acid, 1,6-hexamethylene diamine and another kind of aliphatic long-chain dicarboxylic acid as raw materials. However, when preparing semi-aromatic polyamide by means of melt-polymerization, the later-phase reaction temperature will exceed the melting point of the polymer, various side reactions and degradation reaction of polymer are violent, apt to cause worsened polymer color, lowered mechanical strength and degradation of formability.
U.S. Pat. No. 5,663,284 discloses a method for preparing polyamide 6T/66 polymer through the method below: perform primary polymerization with the presence of water and under the condition that the reaction temperature is lower than the melting point of polymer, maintain the pressure by filling water into the high pressure autoclave when discharging, melt-polymerize the prepolymer by means of the exhaust type double-screw extruder to get high viscosity polymer. However, to get the prepolymer to conform to melt-polymerization requirements, the prepolymerization temperature is close to the melting point of polymer so as to improve the inherent viscosity of prepolymer.
Among the existing technologies, U.S. Pat. No. 6,133,406 puts forward a polymerization technique of semi-aromatic polyamide: with the presence of water, synthesize low inherent viscosity prepolymer at low temperature firstly, then prepare relatively high inherent viscosity polymer through solid state polymerization reaction, finally get a relatively high inherent viscosity polymer through melt-extrusion polymerization with a double-screw extruder. This route involves multiple reactions of prepolymerization, solid state polymerization and melt-extrusion polymerization, requires complicated production procedure and equipment.
In U.S. Pat. No. 6,156,869, after obtaining a prepolymer, a long-period solid state polymerization process can be adopted to get polyamide 9T resin. This technology requires prepolymer with relatively high inherent viscosity. Polyamide 9T features relatively high crystallinity, dimensional stability and relatively low water absorption.
Among the existing technologies, the semi-aromatic polyamide prepolymer with relatively high inherent viscosity can be obtained through raising prepolymerization temperature or discharge the water in prepolymerization reaction system. The heightening of prepolymerization temperature may lead to the occurrence of side reactions, and also the increase of reaction pressure, so the requirements for equipment are also upgraded correspondingly. The diamine without reaction will vaporize with water discharged from reaction system, which results in the great difference between the monomer unit ratio of prepolymer and the initial monomer ratio added into the reactor, thus the Mole ratio balance between dicarboxylic acid monomer and diamine monomer is incapable of being guaranteed.
Diamine loss can be avoided through using a monomer to make salt first, then synthesizing semi-aromatic polyamide by polycondensation of the salt. U.S. Pat. No. 5,663,284 discloses the preparation of semi-aromatic polyamide by using salt through measuring PH value to determine the end point of salt reaction. However, there is a problem in the existing technology: when preparing semi-aromatic polyamide salt by using water or alcohol as a solvent, since the solubility of aromatic dicarboxylic acid and semi-aromatic polyamide salt in these solvents is low, it is difficult to purify semi-aromatic polyamide salt and further get pure semi-aromatic polyamide salt, thus the Mole ratio of dicarboxylic acid and diamine is incapable of being controlled accurately, affecting the quality of the resultant semi-aromatic polyamide; when preparing semi-aromatic polyamide salt by using other organic solvents, high-purity semi-aromatic polyamide salt can be got, but the recycling costs of solvent are heightened.
The method for treating wastewater generated during the synthesis of semi-aromatic polyamide is not involved in the abovementioned patents.